Reptiles tend to be very conservative in karyotype, and yet have evolved a variety of mechanisms for sex determination, including genotypic sex determination of the XX/XY type, genotypic sex determination of the ZZ/ZW type, and environmental sex determination, where sex is determined by an environmental stimulus, such as incubation temperature during development. The proposed experiments have two main goals: a comparison of the molecular bases of different sex determination mechanisms in reptiles; and the use of DNA sequence data to construct phylogenetic relationships among certain groups of pit vipers. Southern blot hybridization will be used to look for sequences homologous to the mammalian sex determination gene, Sry, in genomic DNA from pit viper with genotypic sex determination, and from the desert tortoise, with environmental sex determination. If homologous sequences are detected, it will suggest that all mechanisms of sex determination are similar at the genetic level; this may contribute to our understanding of sex chromosome aneuploidy syndromes in humans. The pit vipers are responsible for a large fraction of snakebite injuries and fatalities. The proposed phylogenetic investigations might aid in the rational production and use of antivenins, and in the isolation of biomedically useful products. Student participants will gain a background in molecular genetics, and direct experience in performing a variety of techniques of recombinant DNA and general nuclei acid manipulation. After a training period, they will have the opportunity to conduct independent research, and thereby prepare for further graduate studies leading to a Ph.D. degree, and for careers as competitive biomedical research scientists.